Originally I went from a ZR 600 to an M6 and it was quite the difference. You will get a lot of different opinions on the trail manners of the M, but I really liked the seat position and overall I thought it was a good trail sled as well as off-trail. The Crossfire is probably a bit more well rounded and has a better balance of on trail and off trail capability. The 600 laydown motor is getting a little long in the tooth, it makes decent power, but not compared to a new 600 Poo or the 600 Doo ETEC. This motor has a thirst for fuel as well, probably a little better than the 600 case reed motor you currently have. There were a couple reports in the mags of the 600 motor making more HP in 2010, but unless there was some ECU mapping changes, nothing has really changed with the motor.
Depending on your budget you may want to consider the 800 over the 600, just for the new motor.
Stack